1. Field of the Invention
The invention relates generally to regenerative heat exchangers, and in particular, to a gas permeable refractory brick for use therein. The invention also relates to a hot grid formed from the refractory brick.
2. Description of the Related Art
Regenerative heat exchangers operate by passing a stream of a relatively hot gas through a heat exchange mass during one period (gas phase) to store heat in the mass. A stream of a relatively cool gas is subsequently passed in the reverse direction through the mass during a second period (blast phase) to recapture this stored heat. With heat exchangers of this type, it is customary to have the gas phase and the blast phase alternately recur, and to provide at least two heat exchange masses. In this way, while heat is being stored in one of the masses, heat can be recovered from the other mass. The refractory brick lined hot stove used in the iron making industry to feed blast furnaces with hot blast is one such example of a regenerative heat exchanger.
The so called xe2x80x9cpebble bed regenerative heat exchangersxe2x80x9d are typically cylindrical in structure, and include a heat accumulation mass which consists of a loose bulk material arranged in a space and held in place between two concentric walls (i.e., an inner hot grid and an outer cold grid) which are permeable to gases. A hot collection chamber is circumscribed by the inner hot grid for collecting the hot gases. A cold collection chamber for collecting the cooled gases is typically defined by the space between the outer cold grid and the external wall of the regenerator.
A regenerator of the above-described type is disclosed in U.S. Pat. No. 2,272,108, to Bradley. The quantitative embodiment described in that document, however, cannot operate in practice. The gas speed selected for passing through the heat accumulation mass is much too small while the size of the particles making up the loose bulk material of the heat accumulation mass is too large. This results in an inadequately small head loss of the gas in the material bed. The pressure of the gas thus decreases with height in the cold collection chamber. This effect, known as the xe2x80x9cstack effectxe2x80x9d, is negligible in the hot collection chamber. The pressure difference caused by the stack effect is a multiple of the pressure drop in the material bed. Consequently, when heating the regenerator, the heating gases flow only in the upper region through the material bed. Backflow of the gases might even be expected in the lower region. When working under hot blast, i.e., during cold blowing, the conditions are reversed. That is to say that only the lower region of the material bed would be exposed to the gases. These results lead to the conclusion that the regenerator described in this document would necessarily fail.
A further problem associated with the heat exchanger design of conventional hot grid structures is their tendency to accumulate dust, thereby inhibiting flow of the gas therethrough during the blast and gas phases. This results in an increase in pressure drop through the brick and heat accumulation bed.
The main concern regarding dust loading of the gas stream is plugging of the openings of the bricks in the grid, as well as sticking of the particles in the heat accumulation bed. It has been found that particles in direct proximity to the hot grid openings tend to become coated by a hard, sintered layer of dust. This dust layer acts as a cement, binding the particles together in the regions close to the hot grid openings. As a result, the porosity of the heat accumulation bed becomes decreased, and the pressure drop through the bed increases. This phenomenon is particularly detrimental to the heat transfer efficiency of the heat exchanger.
Moreover, the high operating temperatures and thermal cycles experienced by the hot grid place extreme demands on that structure. In this respect, the succession of blast phase and gas phase cycles submits the hot grid to repeated stress cycles. The mechanical stress under which the bricks and hot grid can operate is generally limited by its weak point. Such a weak point typically occurs each time an important structural change in the brick occurs. The junction between the structures is often a potential crack development location.
U.S. Pat. No. 5,577,553, to Fassbinder discloses a hot grid made up of individual bricks composed of a heat resistant material, such as ceramic. The bricks have a cavity which opens into an annular chamber containing the heat-storage medium. The cavity is filled with pellets which are mutually consolidated and secured against dropping out of the brick by a heat resistant adhesive. A blind-hole bore, starting from the wall of the brick adjacent to the hot collecting chamber enclosed by the hot grid, extends into the cavity filled with the pellets. The disclosed brick, however, is disadvantageous in that its structure is complicated and is made of numerous pieces. The brick is thus more subject to stress build up and breakage is possible, especially at the junction between pellets and between pellets and brick. The adhesive material which glues the pellets together must withstand high stresses. Moreover, the production of such a brick is not easy and induces high costs.
To avoid or conspicuously ameliorate the problems associated with the state of the art, it is an object of the present invention to provide gas permeable bricks of a refractory material suitable for use in a hot grid of a regenerative heat exchanger.
It is a further aspect of the invention to provide hot grids suitable for use in a regenerative heat exchanger formed from a plurality of the inventive bricks.
The bricks and grids in accordance with the invention allow the gases during the gas and blast phases to flow freely therethrough, resulting in a significantly lower pressure drop through the heat accumulation mass than has been possible to date. The inventive bricks and grids allow for improved distribution of the hot gas in the heat accumulation bed next to the hot grid such that flow rate and other characteristics of the gas depend only on the radius of the point at which it is measured in the bed, and not on the height of the bed or the angle of flow. At the same time, the bricks and grids in accordance with the invention provide mechanical support to the loose bulk material of the bed, are more resistant to heating and are less costly to manufacture than known bricks.
Provided are novel gas permeable bricks of a refractory material suitable for use in a hot grid of a regenerative heat exchanger. In accordance with a first aspect of the invention, the brick has an inner face and an outer face on opposite sides of the brick. One or more cavities extend from the inner face partially into the brick. A plurality of channels for each of the cavities extend from the outer face to the cavities. The cavities and channels allow a gas to pass through the brick.
The brick preferably has a shape which is a sector of a circle, the inner face facing the center of the circle and the outer face forming the periphery of the circle. The cavities are preferably symmetrically distributed over the inner face of the brick, and extend into the brick from the inner face about one half to two thirds the length of the brick, measured from the inner face to the outer face. The brick typically has a ratio of open area:closed area at the inner face of from 0.1:1 to 0.5:1, and a ratio of open area:closed area at the outer face of from 0.1:1 to 0.5:1. The brick can additionally include a plurality of grooves in the outer face overlapping the channels. The grooves typically extend from 2 to 15 millimeters into the outer face. The brick is preferably constructed from a single material and from a single piece of the material.
In accordance with a further aspect of the invention, a gas permeable brick of a refractory material suitable for use in a hot grid of a regenerative heat exchanger is provided. The brick has an inner face and an outer face on opposite sides of the brick. A plurality of channels extend through the brick from the inner face to the outer face. The channels allow a gas to pass through the brick.
The brick typically has a shape which is a sector of a circle, the inner face facing the center of the circle and the outer face forming the periphery of the circle. Preferably, the ratio of open area:closed area at the inner face and the outer face is from 0.1:1 to 0.5:1. The channels typically have a cross-sectional area of from about 4 to 500 cm2, and the cross-sectional area can be substantially constant along the length thereof. A top face and a bottom face of the brick each can have at least one horizontal section and at least one non-horizontal section having a non-zero slope with respect to the horizontal section in a direction from the outer face to the inner face. The slope of the non-horizontal section is typically greater than 15xc2x0 from horizontal. Preferably, the brick is constructed from a single material and from a single piece of the material.
In accordance with further aspects of the invention, hot grids suitable for use in a regenerative heat exchanger formed from a plurality of the bricks are provided.